兩座超大陸之間的關聯

原文網址：www.sciencedaily.com/releases/2016/04/160412091348.htm

Links within two supercontinents

兩座超大陸之間的關聯

A University of Wyoming researcher contributed to a paper that has apparently solved an age-old riddle of how constituent continents were arranged in two Precambrian supercontinents -- then known as Nuna-Columbia and Rodinia. It's a finding that may have future economic implications for mining companies.
懷俄明州立大學的一位研究人員撰寫的論文似乎解開了一個年代久遠的問題：前寒武紀的兩座超大陸—哥倫比亞大陸(Nuna-Columbia)和羅迪尼亞大陸(Rodinia)的位置是如何分布的。對礦業公司而言這項發現未來或許具有經濟上的潛在利用價值。

Specifically, the article describes a technique Kevin Chamberlain, a UW research professor in the Department of Geology and Geophysics, and other researchers used to test reconstructions of ancient continents. The paper argues that the rocks or crust now exposed in southern Siberia were once connected to northern North America for nearly a quarter of the Earth's history. Those two continental blocks now form the cores of the modern continents of Asia and North America.

具體來說，這篇論文內容記述的是懷俄明州立大學地質與地球物理學系的教授，Kevin Chamberlain和其他研究人員用來重建這兩座古代大陸時所用的技術。這篇論文主張現今在西伯利亞南方出露的岩石或地殼曾跟北美北方相連，時間長達將近地球歷史的四分之一。而現今這兩個古陸塊則構成了亞洲大陸和北美大陸的核心。

Chamberlain was co-author of the paper, titled "Long-Lived Connection between Southern Siberia and Northern Laurentia in the Proterozoic," that appeared in today's (April 11) online issue of Nature Geoscience. The monthly multi-disciplinary journal focuses on bringing together top-quality research across the entire spectrum of the Earth sciences, along with relevant work in related areas. The journal's content reflects all the disciplines within the geosciences, encompassing field work, modeling and theoretical studies.

這篇題名為「原生代時西伯利亞大陸南部和勞倫大陸北部之間曾相連許久(Long-Lived Connection between Southern Siberia and Northern Laurentia in the Proterozoic)」的論文刊登在今日(4月11日)的《自然—地質科學》(Nature Geoscience)線上版，共同作者之一為Chamberlain。這本跨領域的科學月刊著重在彙整地球科學各分野中品質最佳的研究，以及其他有關領域中與地球科學相關的研究成果。此期刊的內容呈現出地質學家進行研究時會用到的各種方法，包含野外調查、電腦模擬及理論方面的研究。

"The article highlights a technique that our project has been using to test pre-Pangea or ancient continental reconstructions," Chamberlain says. "We have been using the ages, orientations and paleo-magnetic characteristics of short-lived (1 million to 10 million years in duration) igneous, mafic dike swarms as piercing points to determine nearest-neighbor continents in the past."

「此篇文章的內容側重在我們計畫中用來嘗試重建前盤古大陸以及古代大陸時用的方法。」 Chamberlain說。「我們將在短時間內(100萬至1000萬年)形成的基性火成岩脈群作為銜接點，並利用它們的年代、方位及古地磁特性來找出在過去曾緊密相連的大陸。」

Mafic dikes are dark-colored rocks or minerals that are in a dike formation, which is a sheet of rock that formed in a fracture in a pre-existing rock body. Chamberlain says mafic dikes, like those studied in the paper, can be found in Wyoming. Mafic dikes in the state include the black vein that can be seen in Mount Moran in the Teton Range; the black, horizontal band on the east face of Medicine Bow Peak; and those that crisscross the Granitic Mountains in central Wyoming.

基性岩脈是以脈絡狀分布的暗色岩石或礦物，這種岩體通常形成於更早之前即存在的岩石的裂縫之中。 Chamberlain說在懷俄明州境內可以找到像是這篇論文中研究的基性岩脈。此州境內有基性岩脈分布的地點包含了提頓山脈莫蘭山中的黑色岩脈、梅迪辛波峰東面的黑色水平帶狀岩石，以及懷俄明中部花崗岩山峰中縱橫交錯的岩脈。

Using labs at UW and UCLA, Chamberlain says his role in the project was to determine the magmatic ages of numerous mafic dikes through uranium-lead radiometric dating. He was one of four geochronology labs on the team and the only one based in the United States.

Chamberlain說他在這項研究計畫中的腳色是利用懷俄明州立大學和加州大學洛杉磯分校的實驗室，透過鈾鉛放射性定年法分析大量的基性岩脈以確認個別岩體的岩漿形成年代。他是這個團隊中四位地質定年學家中唯一在美國進行實驗的人員。

The linear dikes from these igneous events (large igneous provinces, or LIPs) are relatively narrow, roughly 100 meters or less, but can be 1,000 to 1,500 kilometers in length. They erupt in a radial pattern.

這些岩漿事件(大火成岩省(large igneous provinces, LIPs))形成的岩脈長度可以長達1000至1500公里，然而寬度相對而言並不寬，僅約100公尺甚至更窄。它們通常以輻射狀的模式噴發。

During later rifting, the continents broke into fragments, which later combined into subsequent new continents, such as our modern-day seven continents.

大陸在往後的張裂作用中會被撕成碎片，然而這些碎片之後又會再次組合成為新的大陸，好比說今日世上的七座大陸。

"There may have been four or five cycles of supercontinent formation," Chamberlain says.

「超大陸的形成循環可能已經發生了4或5次之有。」Chamberlain 說。

Each continental fragment preserves a dike swarm record, he explains. By comparing the temporal records called bar codes (since a plot of dike date vs. time looks like a bar code) of older fragments known as cratons (the cores of modern continents), Chamberlain says he was able to test whether the cratons were close enough to share LIP dike swarms. He adds the research team also can determine when the two cratons joined, as well as when they split apart.

他解釋大陸的每一塊碎片都保有自身的岩脈群記錄。藉著比對不同塊古老大陸的碎片，即穩定地塊(又稱克拉通(craton)，為現今大陸的核心)內這些稱作「條碼」的紀錄(這是因為岩脈年代對時間的作圖看起來就像條碼)，Chamberlain 說他可以驗證這些穩定地塊在當時是否相當靠近，而擁有相同的大火成岩省岩脈群。他補充說明研究團隊同樣可以依此來得知這兩座穩定地塊是何時相連，又是何時分開。

"In this new study, we believe that northern Laurentia (North America) and southern Siberia were joined for nearly 1.2 billion years from 1.9 billion years ago to 700 million years ago," he says. "Geologists are like detectives. It seems like we come to the crime scene after the fact and put together the pieces."

「在這項新研究中，我們認為勞倫大陸北部(北美)和西伯利亞大陸南部曾經相連長達將近12億年之久，約從19億年前至7億年前。」他說。「地質學家的工作就像是在案件發生後到達現場，將殘缺的證據拼湊出真相。從這點來看我們跟偵探可說是十分相似。」

This finding disproves previous constructions of Nuna-Columbia and Rodinia, and establishes new arrangements of the continental blocks within them, he says.

他說這項發現跟先前哥倫比亞大陸和羅迪尼亞大陸的重建結果相悖，而他們也將這兩塊大陸內部的陸塊以新的方式重新排列。

The project determined the ages of nearly 250 mafic dikes worldwide, a number Chamberlain says is large enough to build a database comparison between all of the older continental fragments from roughly 500 million years ago to 2,700 million years ago. The research group also worked on more recent LIPs -- about 400 million to 100 million years ago -- which have importance for oil and gas exploration, and hydrocarbon maturation models.

這項計畫定出了世界各地將近250條基性岩脈的年代。 Chamberlain說這麼龐大的數目足夠他們建立起一個資料庫，以比對所有較為古老，年代約從5000萬年前至2億7000萬年前的大陸地殼碎片。研究團隊也探討了年代更近，約從4000萬年前至1000萬年前的大火成岩省事件。這些事件對油氣探勘以及碳氫化合物熟成模型的研究上皆具有相當重要的意義。

A consortium of mining companies funded the research project for five years. Their reasoning: That the continental reconstructions for times when major, known metal deposits formed would be useful for prospecting new finds on the conjugate continents, Chamberlain says. These new deposits may be buried under hundreds of meters of younger rock. So, by establishing which continents were next to the known deposits when they formed, the hope is that additional minerals may be found in the future.

一個礦業財團資助這項計畫長達5年的時間，據Chamberlain說他們的理由為：「重建大型已知礦脈形成時的陸地樣貌，將有助於從過往結合在一起的其他陸地上探勘出新礦藏。」這些礦藏可能深埋於數百公尺厚的較年輕岩石之下，因此若能重建出在已知礦脈形成時與它毗鄰的陸地位置，那麼就有望在未來可以在此發現新礦藏。

"A lot of the major metal deposits in the earth formed in the early part of Earth's history," Chamberlain says.

「許多重要的金屬礦床都是在地球歷史相當早期形成的。」Chamberlain說。

引用自：University of Wyoming. "Links within two supercontinents." ScienceDaily. ScienceDaily, 12 April 2016.